How severe is CVE-2021-30477?

CVE-2021-30477 has a CVSS 3.x base score of 4.3, rated medium severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is none, integrity low, and availability none.

Is CVE-2021-30477 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (47th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-30477?

CVE-2021-30477 affects Zulip Zulip Server. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-30477?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

When was CVE-2021-30477 published?

CVE-2021-30477 was published on 2021-04-15 and last updated on 2026-06-17.

CVE-2021-30477

CVE-2021-30477 is a medium-severity vulnerability in Zulip Zulip Server with a CVSS 3.x base score of 4.3. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low.

Key facts

Severity: Medium (CVSS 3.x base score 4.3)
CVSS v2: 4.0
EPSS exploit prediction: 1% (47th percentile)
Actively exploited: Not listed in CISA KEV
Affected product: Zulip Zulip Server
Published: 2021-04-15
Last modified: 2026-06-17

Description

An issue was discovered in Zulip Server before 3.4. A bug in the implementation of replies to messages sent by outgoing webhooks to private streams meant that an outgoing webhook bot could be used to send messages to private streams that the user was not intended to be able to send messages to.

Frequently asked questions

What is CVE-2021-30477?: An issue was discovered in Zulip Server before 3.4. A bug in the implementation of replies to messages sent by outgoing webhooks to private streams meant that an outgoing webhook bot could be used to send messages to private streams that the user was not intended to be able to send messages to.
How severe is CVE-2021-30477?: CVE-2021-30477 has a CVSS 3.x base score of 4.3, rated medium severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is none, integrity low, and availability none.
Is CVE-2021-30477 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (47th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-30477?: CVE-2021-30477 affects Zulip Zulip Server. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-30477?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
When was CVE-2021-30477 published?: CVE-2021-30477 was published on 2021-04-15 and last updated on 2026-06-17.

References

https://blog.zulip.com/2021/04/14/zulip-server-3-4/

Affected products (1)

cpe:2.3:a:zulip:zulip_server:*:*:*:*:*:*:*:*

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